1. Field of the Invention
The present invention relates to a video projector for displaying an image according to television signals and the like and, more particularly, to a video projector using an active matrix liquid crystal light valve.
2. Description of the Prior Art
Conventionally, in video projectors made for home use there is employed a CRT as is seen in the case of rear projection TV. Video projectors of the CRT system have a more or less satisfactory degree of display quality with respect to images of such resolution level as presently available TV signals, but still it can hardly be said that they are capable of displaying images with sufficient brightness. Further, if they were to display images of such high definition TV as would be a reality in the near future, it would be almost impracticable to expect that the required display quality for the purpose could be provided at some reasonable cost, in view of those serious problems which will be pointed out hereinbelow. Whereas the degree of resolution available with the existing TV system is of the order of 500 .times.600=300,000 pixels, that of the would-be high definition TV will be 1000.times.1500=1,500,000 pixels, or about five times as high as the presently available TV image resolution.
A first problem with the CRT system video projector is that there is some tradeoff between the brightness and the resolution on the faceplate of the CRT, and further that luminous rays from the CRT tend to spread over a wide area, so that the available quantity of light is limited. Therefore, video projectors using a CRT require a very costly projection lens with large aperture in order to attain improved efficiency of light utilization, but yet it is difficult that they provide sufficient brightness on the screen.
A second problem with the CRT system video projector is that distortions occur in the trace of the electron beam on the surface of the CRT, with the result that some distortion is present in an image on the CRT surface. Therefore, with a full color video projector of the three-tube CRT system, high-precision adjustment and matching measures are required for convergence correction of each CRT and otherwise. Such adjustment becomes difficult in abrupt proportion as the required degree of resolution of the to-be-displayed image becomes higher.
As an alternative to the CRT system, light valve system projectors have already been proposed. For example, a photoconductive-type liquid crystal valve has been reported in the 1979 SID international Symposium, Digest of Technical Papers, pages 22 to 23. By the term "light valve" is meant a device which is capable of changing, or modulating, the properties of light rays incident thereon, such as their amplitude and polarizing plane, as required by externally given signals, then projecting the modulated light rays; and herein the term specifically refers to such device of the type which can two-dimensionally control or modulate rays of incidence for image formation. A projector of this system includes a light source, a light valve, a projection lens, and a screen, wherein light rays from the light source are modulated by the light valve for formation of a light image, the light image so formed being projected through the projection lens onto the screen.
However, the state of the art of light valve system projectors is still insufficient for commercialization of such projectors for home use. Nevertheless, one type of light valve is recently attracting great attention, that is, light valves of the type which utilize liquid crystal and, more particularly, those known as liquid crystal light valves of the active matrix system are considered promising for their potentiality of providing high picture quality. Typical examples of such system are those disclosed in Japanese Patent Publication Nos. 59-230383 and 61-13885.
The term "active matrix system" used herein means a liquid crystal driving scheme as so called in comparison with the conventional simple matrix scheme, wherein switching elements are individually provided for pixel electrodes arranged in a matrix pattern, through which switching elements driving signals are given independently to the individual pixel electrodes for controlling the optical properties of the liquid crystal. A liquid crystal light valve of this system, as will be described hereinafter, has a feature that it is essentially free of such crosstalk as is usually seen with the conventional simple matrix scheme, or a phenomenon such that electrical signals supplied to different pixel electrodes intermingle with one another, which will be a cause of display quality deterioration; therefore, the light valve can display large-volume information while maintaining high contrast and good image quality.
Now, in order to facilitate the understanding of the following description, a basic construction of an active matrix system liquid crystal light valve and the manner of operation thereof will be briefly explained.
The liquid crystal valve comprises an array substrate, a counter substrate having a counter electrode, and a liquid crystal layer interposed between the two substrates. On the array substrate there is formed such an active matrix array circuit 1 as shown in FIG. 1. The array circuit consists of a plurality of scanning lines Xn (n=1, 2, . . . , N), a plurality of signal lines Ym (m=1, 2, . . . , M), pixel electrodes disposed at intersections between both the scanning and the signaling lines, and switching elements Tn, m connected to the corresponding pixel electrodes and controlled by the corresponding scanning lines. In FIG. 1, each switching element is a thin film transistor.
In operation, the signal lines have a role of leading video signals supplied from an externally provided video signal supply circuit 2, to the individual switching elements. The counter electrodes keeps a common reference level for the video signals, or acts as a ground. Selection pulses S1, S2, S3, . . . , SM as shown in FIG. 2, which do not overlap in time, are sequentially applied as control signals to the scanning lines X1, X2, X3, . . . , XN respectively from a control signal generating circuit 3. When a selection pulse is applied to one, e.g. X1, of the scanning lines (i.e. X1 is in a "selected phase"), a series of switching elements connected to the line become conductive to transmit a video signal to the corresponding series of pixel electrodes. At this time, all the other scanning lines are in a "non-selected phase". Voltages applied to the series of pixel electrodes are held until the scanning line X1 is selected next time. The voltage holding capability depends on the capacitance Cn, m present between each pixel electrode and the counter electrode.
The video signal applied to each pixel electrode controls the electro-optical property of the liquid crystal layer interposed between the pixel electrode and the counter electrode, and more particularly the polarizing angle of the light rays which pass through the liquid crystal layer. The electro-optical property of the liquid crystal layer is by nature determined by an effective voltage applied to the layer. As already explained, an effective voltage applied to the liquid crystal layer for a given pixel is generally equal to a signal voltage which is transmitted to the corresponding pixel electrode from a signal line through a switching element when a selection pulse is applied. With the active matrix system, it is possible to independently control the electro-optical properties of the individual pixels in this way.
In order to enhance the performance of the liquid crystal light valve for holding the voltage applied to each pixel electrode, it is more preferable to provide each pixel electrode with an additional capacitance Cn, m, S. Such additional capacitance may be sometimes provided between each pixel electrode and a separately formed common electrode, but the trouble in such case is that increased process steps are involved in the formation of the array substrate. Therefore, it is more common to provide the additional capacitance between a preceding scanning line Xn-1 and a pixel electrode Pn, m, as shown in FIG. 1.
However, the video projectors of the liquid crystal light valve system which have thus far been proposed still involve a number of problems such as those stated hereinbelow.
In a video projector using a liquid crystal light valve, how far the light valve could be compacturized has great bearing upon the cost of the optical system and accordingly the cost of the projector system. Therefore, in the area of application which require large-volume information display as in the case of high definition TV, a light valve having a very high degree of pixel density is required. In other words, dimensions (areas) allotted to individual pixels of the light valve are very small. Whilst, the region for arrangement of signal lines, scanning lines, and switching elements occupies a generally fixed area irrespective of the space required for the pixels. This region has to be counted as a dead area which does not contribute to the control of light rays. Therefore, the proportion of an area which can be utilized for effective control of light rays of the area allotted to one pixel (which proportion is called "opening") is abruptly reduced as the size of the pixel becomes smaller. This naturally means reduced efficiency of light utilization. This is pointed out as a first problem.
The light valve is constantly exposed to a very intense illumination of light beams. The light beams induce photoconduction of the individual switch elements connected to the pixel electrodes, which is likely to be a cause of lowered image quality. The reason for this is that where there is some photoconduction, any of the switch elements may be brought into a slightly conductive state even if it is in the non-selected phase, with the result that the voltage applied to the pixel electrodes cannot be sufficiently held. Since each switch element is formed of a semiconductor or a very thin insulating material, it is susceptible to the influence of light beams. This is a second problem.
A third important problem with a high-density liquid crystal light valve intended for large-volume information display is a low production yield of such light valve. On factor having the greated bearing upon production yield is the presence of defective switch elements. If there is any defective switch element, the pixels connected to the switch element does not perform their proper display function, which is a direct cause of the so-called display defect. From a statistical point of view, as the earlier given example may suggest, it is extremely difficult indeed to produce as many switch elements as 1,500,000 defect free.
As far as this problem is concerned, we have already proposed, in our previous U.S. patent application Ser. No. 798,030, an approach for overcoming the difficulty relating to production yield by adoption of such a redundancy arrangement that two or more switch elements are provided for each pixel as shown in FIG. 3, so that defective switching elements, if any, can be removed by trimming. For the following two reasons, however, it is difficult to immediately apply this approach to a light valve having a very high degree of pixel density to which the present invention is directed.
One reason is the increased number of switch elements per pixel, which results in reduced opening.
The other reason is the problem of capacitive coupling due to the additional capacitance, which will be explained below. FIG. 4 illustrate an array circuit representing the above proposed arrangement in which additional capacitances are provided. Attention is called to a pixel electrode Pn, m. In the case where the switch element Tn,m,A is separated because of its defective quality, there will be no problem. But if the switch element Tn,m,B is removed, the change of the control signal applied to the scanning line Xn1 from selection to non-selection, is transmitted to the pixel electrode through the additional capacitance Cn,m,S, so that the potential of the pixel electrode deviates from its should-be value. This means that despite the fact that the defective switch element has been separated, the pixel electrode does not perform its proper display function.
As explained above, the prior art liquid crystal light valves are not properly adapted for provision of higher pixel density, and are much liable to adverse effects of the illuminated light beams. Further, there is no effective means for improvement of production yield. As such, they are unsuitable for use in a video projector for large-volume information display.